Good discussion here. I've been able to do quite a bit of subjective testing between light 40 S&W loads vs. 9mm loads in the same gun. I have an M&P 40 along with a 9mm barrel and magazines. The main point of recoil as mentioned earlier is momentum exiting the barrel. That is the weight of the bullet times the velocity of the bullet. There is also the speed and mass of the powder burn products exiting the muzzle that play into the recoil but for "felt recoil" the bullet weight and speed are close enough.
My typical 9mm load is 120g bullets at 1100 fps which gives me an IPSC power factor of (120g x 1100fps)/1000 = 132
My 40 S&W cast bullets are 180g in weight so to achieve a 137.5 power factor, I need to make a load that pushes the bullets (132 x 1,000) /180g = 733 fps
After testing numerous powders and charges, I found that 2.7g Clays gives me 735 fps.
I shot a lot of round going back and forth between 1100 fps 120g 9mm loads and 735 fps 180g 40 S&W loads using the same gun and only changing the barrel and magazines. As far as I can tell, the loads shot the same with the exception of the hole size on the paper at 10 yards.
I typically load 180g 40 S&W at 950-1000 fps but have a number of boxes loaded with 3.0g Clays or Red Dot for "light loads" to let my kids shoot the 40 with close to 9mm recoil (just a little stronger to ensure good cycling and lockback of the slide after the last shot.
For economy's sake, if I really want to shoot a bunch of 9mm recoil loads, I'd rather load 9mm cartridges. One extra grain of powder for the 9mm load is cheaper than 60 grains of lead going from 120g to 180g (1/2 cent per shot)